The temperature of a comfort zone, such as a room within a building, can be controlled by regulating the amount of temperature conditioned air supplied to the zone. Airflow regulators used for this purpose are mounted to a supply air duct and are generally referred to as variable air volume valves, or simply VAV valves.
Airflow through VAV valves is often controlled by varying the valve opening in response to the temperature of the zone. Valves under such control are referred to as pressure dependent valves, because for a given valve opening, the amount of airflow depends on the air pressure upstream of the valve. In some systems, airflow regulation becomes inadequate as a result of widely varying upstream pressure due to varying supply air blower speed or the effects of the opening and closing of other VAV valves in the system.
An improvement in airflow regulation is provided by controlling VAV valves in response to upstream pressure in addition to zone temperature. Such valves are referred to as pressure independent valves. Should the upstream pressure vary for any reason, the pressure independent valve will compensate by opening or closing an appropriate amount to maintain the desired airflow.
Pressure independent valves are generally superior to pressure dependent valves, provided an airflow indicator associated with the pressure independent valve doesn't fail. Should failure occur, present VAV valves typically lock at a fixed position. Depending on the specific control, the valve may lock fully open, fully closed, or at some other intermediate position. Regardless of the position, the failure of a pressure transducer associated with the airflow indicator destroys the valves ability to modulate flow.
Therefore, it is an object of the invention to provide a VAV valve that modulates airflow in response to temperature and upstream pressure, and continues to modulate airflow even when a pressure transducer associated with the valve fails.
Another object of the invention is to provide a VAV valve with a control that avoids the flow regulating problems associated with pressure dependent valves.
A further object is to reduce a VAV valve's dependence on a flow indicator incorporating a pressure transducer.
A still further object is to provide a VAV valve that properly modulates airflow as it compensates for varying speeds of an upstream supply air blower and compensates for the opening and closing of other VAV valves.
Another object is to detect a faulty pressure transducer by determining the valve position and comparing a signal provided by the transducer to a predetermined normal range for the given valve position.
Yet another object is to determine an intermediate valve position between fully open and fully closed without actually sensing an intermediate position of the valve.
These and other objects of the invention will a apparent from the attached drawings and the description of the preferred embodiment that follows below.